The present invention concerns a device for the transmission of information by pulse code modulation. It also concerns a use of this transmission device.
Pulse code modulated transmissions (PCM) have long been known and applied with advantage. The signal to be transmitted, called a message, is constituted by binary digits selected from the alphabet "O" "I". It is obtained either directly from a source of digital information (keyboard, computer, etc) or by digitalisation of a similar continuous signal. This representation of the message is called PCM.
The message may be transmitted by directly modulating a carrier. For technical reasons it is sometimes necessary to pre-modulate at first a sub-carrier.
The carrier or sub-carrier normally may be frequency or phase modulated. But as the modulating signal is a digital message, these particular techniques such as frequency or phase shift modulation are preferred. Both types of modulation have their applications: In this case we are interested only in frequency shift modulation.
Frequency shift modulation is usually referred to by the sign FSK which comes from the English term "Frequency Shift Keying". In this process, the message m(t) modulates the frequency of a carrier or a sub-carrier for giving a message m* (t) in which the digits " " and "1" are characterised by two frequencies f.sub.o and f.sub.1 attributed to the carrier or sub-carrier.
The frequency shift may be defined as the difference of the two frequencies f.sub.o and f.sub.1. It is necessary to distinguish, at least theoretically, between coherent and incoherent modulation. In the first case, the phase of the carrier or sub-carrier is the same for all the digits of the same value in the message. In the second case, it varies as a function of the position of the digits. Incoherent modulation is more used in practice and the process of the present invention is associated with this category.
In addition, the phase-locking device has long been known and used for receiving and filtering signals.